The present invention relates to a process for reacting hydrogen sulphide and sulphur dioxide gases to produce elemental sulphur and water vapour.
The process is useful in the removal of hydrogen sulphide from a natural gas or industrial gas source (which may include gases produced from a hydrodesulphurization unit of an oil refinery). For example, gas wells presently exist which produce so called "sour gas". Sour gas is a term applied to the products of gas wells, or other industrial gas sources which contain hydrogen sulphide, usually in fairly low concentrations. Processes for removing hydrogen sulphide from sour gas or from high sulphur petroleum products, natural gas, coal, tar sands, heavy oils and fuel oil generally result in the production of an acid gas stream in which the hydrogen sulphide is concentrated and which may contain other gaseous ingredients such as carbon dioxide, water vapour, ammonia, and other impurities. Removal of hydrogen sulphide from acid gases before discharge into the atmosphere is required for environmental and safety reasons.
Conventionally, a product to be treated containing hydrogen sulphide such as a sour gas is passed through a scrubbing unit where hydrogen sulphide is absorbed, and the scrubbing liquid is then regenerated to produce an acid gas stream at substantially atmospheric pressure. The hydrogen sulphide in the acid gas is then converted to water vapour and elemental sulphur by an oxidation process generally known as the Claus process which is a low pressure process involving the following general reaction: EQU 1/2O.sub.2 +H.sub.2 S.fwdarw.H.sub.2 O+S (1)
This general reaction is usually performed in two stages in a process known as the modified Claus process. First a portion of the hydrogen sulphide in the acid gas stream is burnt with air in a combustion furnace as follows: EQU H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (2)
The remaining hydrogen sulphide is then reacted with the resulting sulphur dioxide in a catalytic zone over a suitable Claus catalyst, such as activated alumina, as follows: EQU 2H.sub.2 S+SO.sub.2 .revreaction.2H.sub.2 O+3/n S.sub.n ( 3)
where S.sub.n is a complex molecular mixture of elemental sulphur vapour; n has a value between 7 and 8. The sulphur vapour is recovered by condensation, either in a condenser and a separator, or in a scrubbing tower. The by-product gases from reaction (3) are usually reheated and subjected to additional stages of the catalytic reaction and sulphur condensation.
Reaction (3), hereinafter referred to as the Claus catalytic reaction, in the conventional modified Claus process is performed at about one atmosphere or slightly greater pressures, and at temperatures of about 195.degree.-370.degree. C. It is to be expected from equilibrium theory (Le Chatelier's principle) that an increase in pressure would increase the yield of sulphur from reaction (3), since n is always greater than 3. Hitherto however pressures substantially higher than atmospheric have been avoided since it is well known that even fairly small increases in pressure result in the formation of liquid sulphur which plugs the catalyst and halts the reaction.
It can be shown, based on theoretical and empirical formulae, that if Claus operating pressures are increased and, at the same time, operating temperatures are increased to maintain the system at or above the sulphur dewpoint; then the net effect on reaction yield is negative. Thus, based on conventional operating principles, an increase of system pressure would not be considered a viable alternative to improve process yields.
To avoid condensation of sulphur on the catalyst, after sulphur has been removed in a condenser between catalytic zones, the gases leaving the condenser are reheated before re-entering the catalytic zones. Both the condensing and reheating steps involve an inefficient and expensive gas-liquid or gas-gas heat exchange.
Additionally, sulphur mist is formed in the conventional Claus plant. Because of limitations arising from operating at near atmospheric pressures, efficient removal of sulphur mist is not possible. Thus sulphur mist is often contained in the tail gas which leaves the last catalytic or condensing zone of a Claus plant.
The Claus catalytic reaction is exothermic so that the use of low temperature favors the efficient elemental sulphur production at least when equilibrium conditions apply to the reaction. At atmospheric pressures, temperatures below about 195.degree. C. are desirable for efficient production. These low temperatures, however, are not economical in practice because of the reduced catalytic activity arising at these conditions. Thus, it is preferred to obtain a higher reaction rate by using higher temperatures, even though this reduces the theoretical production obtainable.
Due to the above mentioned considerations, the conventional Claus process is generally limited to about 97.5% to 98.5% recovery of the sulphur contained in an acid gas stream. With environmental regulations indicating a need for at least a 99% sulphur removal efficiency, a more complete sulphur recovery process is needed. For this reason, Claus plants are often operated in tandem with a tail-gas treating unit as described below.
The opinion generally shared by those familiar with the Claus process is that catalytic activity decreases substantially once liquid sulphur is produced in a conventional Claus catalyst bed, e.g., see Claus Kinetics on Alumina, J. H. Blanc et al., Societe Nationale Elf Aquitaine, France, and Aquitaine Co. of Can. Ltd., Proc. of 5th Canadian Symposium on Catalysis, October, 1977.
This opinion is apparent in the design of a number of commercial processes to recover sulphur from a Claus tail gas. In these processes, the tail gas at about one atmosphere pressure is reacted in a catalytic zone at conditions to cause elemental sulphur to condense in the catalyst bed. Once a catalyst bed is loaded with sulphur, the bed must be taken off stream and regenerated. Exemplary of these processes are the Sulfreen process practiced by the Aquitaine Co. of Can. Ltd. at the Ram River Gas Plant, Alberta; and the MCRC Process practiced by the Analine Sulphur Plant at Wood River, Illinois.
The present invention is based on the discovery that, given suitable operating conditions, the Claus catalytic reaction can proceed efficiently at high pressures (between about 5 and about 50 atmospheres absolute) which cause liquid sulphur to condense in the catalyst bed.
In accordance therefore with one aspect of the invention, in a sulphur producing process in which hydrogen sulphide (H.sub.2 S) and sulphur dioxide (SO.sub.2) gases are continuously introduced into a reactor containing a bed of a catalyst which causes these gases to react to produce elemental sulphur, the gases when in the reactor are maintained at such conditions of temperature and pressure that free water in the reactor exists only as water vapour and sulphur is condensed in the catalyst bed, the sulphur being continuously removed from the bed as a liquid, and the pressure in the reactor being maintained at at least 5 atmospheres absolute.
The term "free water" is intended to exclude water chemically combined with or adsorbed onto the catalyst.
We have found that, under the conditions described, the catalyst, even though saturated with liquid sulphur, remains effective. In fact, it is preferred in accordance with this process that the catalyst be substantially saturated with liquid sulphur. It is believed that the catalyst remains active in these conditions because liquid sulphur serves to reduce sulphates on the catalyst. Sulphates are known catalyst poisons in the Claus reaction which are formed on the catalyst during the operation of the conventional process.
Due to the exothermic nature of the Claus catalytic reaction, it is generally desirable to cool the catalyst bed. In accordance with another feature of the invention, the temperature in the catalyst bed is controlled with a liquid coolant, which is preferably liquid sulphur, sprayed on to or otherwise applied to the bed. The temperature of the liquid sulphur being sprayed is in the range of about 120.degree. to 150.degree. C., and the temperature of the bed is such that the gases on leaving the bed (i.e., the "bed exit temperature") have a temperature between about 125.degree. C. and about 375.degree. C., and preferably from 150.degree. C. to 200.degree. C.
The reactor preferably contains several catalyst beds arranged to receive the gases in turn, the liquid sulphur being removed as an individual stream from each bed.
The optimum pressure within the bed is chosen having regard to various parameters including the temperature and the proportion of H.sub.2 S in the incoming gas, and in accordance with ecomomic factors such as the cost of compressing the gas. Increasing the pressure tends to produce more sulphur from the reaction (3) as discussed above and the use of higher pressures reduces the size of the apparatus leading to lower cost. However, it is not thought that pressures above 50 atmospheres (absolute) would be useful. A pressure of about 5 atmospheres is needed to give a 99% conversion of sulphur. However, the normal range of operating pressures is expected to be between 10 and 20 atmospheres absolute.
For particular conditions, suitable pressures may also be calculated for desired conversion and recovery efficiencies. The Claus conversion efficiency (.xi..sub.c) is the fraction of inlet combined sulphur that is converted to elemental sulphur; the Claus recovery efficiency (.xi..sub.R) is the fraction of inlet combined sulphur that is recovered as elemental liquid sulphur. The difference between these efficiencies is primarily due to sulphur vapour losses.
Certain empirical formulae may be used to calculate suitable pressures when other conditions have been determined and assuming a target conversion or recovery efficiency, and also assuming that the H.sub.2 S and SO.sub.2 that enter the reactor are in a substantially stoichiometric ratio.
For example, if a determination is made of a target conversion efficiency .xi..sub.c (P), the pressure may be chosen such that pressure P in absolute atmospheres is at least that given by the formula: ##EQU1## where; EQU .xi..sub.c (1)=1-exp (-6.0698.times.10.sup.-5 T.sup.2 +0.048175T-11.0933+0.282x.sup.2 -0.667x)
and where;
.xi..sub.c (1)=equilibrium fractional conversion efficiency at 1 atm pressure PA1 .xi..sub.c (P)=target, or desired fractional conversion efficiency pressure P PA1 P=reactor pressure, in absolute atm PA1 x=mole fraction of H.sub.2 S in acid gas PA1 T=reactor exit temperature, .degree.C. PA1 0.9&lt;(.xi..sub.c 1)&lt;1.0 PA1 0.2&lt;X&lt;1.0 PA1 160&lt;T&lt;300 PA1 1&lt;P&lt;50 PA1 .xi..sub.c (1)=1-exp (f.sub.c (T,x)) PA1 f.sub.R (T,x)=-2.83.times.10.sup.-5 T.sup.2 +0.05064T-10.4578+0.862x.sup.2 -1.83x PA1 f.sub.c (T,x)=-6.0698x10.sup.-5 T.sup.2 +0.04818T-11.0933+0.282x.sup.2 -0.667x PA1 .xi..sub.R (P)=target, or desired fractional recovery efficiency in the reactor, at pressure P PA1 .xi..sub.c (1)=equilibrium fractional conversion efficiency at 1 atm PA1 P=reactor pressure in absolute area PA1 x=mole fraction of H.sub.2 S in the acid gas PA1 T=reactor exit temperature in .degree.C. PA1 0.9&lt;.xi..sub.c (1)&lt;1.0 PA1 0.2&lt;x&lt;1.0 PA1 160&lt;T&lt;300 PA1 1.0&lt;P&lt;50.0
and where parameters are constrained to the following ranges:
Preferably, the conversion efficiency .xi..sub.c (P) will be at least 0.99 (99%).
Alternatively, if a determination is made of a target recovery efficiency .xi..sub.R (P), the pressure may be chosen such that pressure P in absolute atmospheres is at least that given by the formula: ##EQU2## where; n(1)=exp (f.sub.R (T,x))-exp (f.sub.c (T,x))
and where;
and where the parameters are constrained to the following ranges:
Similarly here, the recovery efficiency chosen will preferably be at least 0.99 (99%).
The factor 1.10 appearing in these formulae is due to the desired pressure being 10% higher than the theoretical pressure to account for lack of complete equilibrium in the reaction.
Pressures may also be chosen according to experimental results. For example, if the process is to be used for treating a gas mixture including a stoichiometric ratio of H.sub.2 S and SO.sub.2 produced by partial combustion of an acid gas containing say from 40 to 100% H.sub.2 S, the pressure may be chosen so that when the bed exit gas temperature is at least 160.degree. C., and with a total residence time in the beds of no more than 15 seconds, at least 99% of the sulphur contained in the acid gas is removed. Preferably, this is achieved with no more than ten catalyst beds.
"Residence time" as used herein means the superficial or apparent residence time, i.e. the time assuming a given size for the bed but also assuming that the catalyst and sulphur occupy no space in this bed; this is conventional.
Alternatively, with the same conditions of temperature and residence time, the pressure may be chosen so that outlet hydrogen sulphide concentration is less than 1500 parts per million and the outlet sulphur dioxide concentration is less than 750 parts per million (by volume).
Preferably, both the liquid sulphur and the gases pass co-currently through the bed. However, counter current or cross flow may also be used.
The catalyst used is preferably an alumina catalyst of the type conventionally used for the Claus catalytic reaction.
The process of this invention can achieve considerably higher recovery efficiency than a conventional Claus plant, eliminating the need for the separate tail gas plant which is required where a conventional Claus plant is operated in an area with strict environmental regulations. Also, the sulphur removal process of this invention can be used with advantage to remove the sulphur from the tail gas of a conventional Claus plant; such a process works continuously unlike in the prior art. Thus, in accordance with a further aspect of the invention, a continuous process for producing sulphur from the tail gas of a conventional Claus plant comprises the steps of: passing the tail gas stream through a compressor and thence at a pressure considerably higher than atmospheric into a reactor containing a bed of catalyst which causes these gases to react to produce elemental sulphur, the reactor being maintained at such conditions of pressure and temperature that free water in the reactor exists only as water vapour and sulphur is condensed in the catalyst bed, the sulphur being removed therefrom as a liquid.
The high pressure reactor which receives the tail gas is preferably arranged to remove enough sulphur that 99% of the sulphur entering the conventional Claus plant is removed before the gas leaves the high pressure reactor. This can be assured by arranging that the high pressure reactor operates at a pressure such that with a bed exit temperature of at least 160.degree. C. and with a residence time of gases in the catalyst of no more than 15 seconds, at least 99% of the total combined hydrogen of the gases entering the high pressure reactor (in stoichiometric ratio) leaves this reactor as water vapour. This is a measure of the conversion efficiency of the overall plant. Since most of the combined hydrogen of the acid gases entering the conventional plant is contained in the H.sub.2 S, if 99% of the combined hydrogen contained in the H.sub.2 S and H.sub.2 O entering the high pressure reactor leaves this as water vapour, then the overall sulphur removal in the plant must be close to 99%.
A high pressure reactor of this kind, receiving the tail gas of a conventional Claus plant, may have only a single bed.
The invention further provides apparatus for reacting hydrogen sulphide and sulphur dioxide gases together to remove sulphur from said gases, including: a reactor containing a bed of a catalyst which causes said gases to react and produce elemental sulphur; supply means suitable for supplying compressed gases including hydrogen sulphide and sulphur dioxide to said reactor at a pressure of at least 5 atmospheres absolute; means for removing liquid sulphur from said bed and for passing the liquid sulphur out of the reactor; and means for cooling said bed so that the conditions of temperature and pressure in the bed allow the compressed hydrogen sulphide and sulphur dioxide to react to produce liquid sulphur in said bed while water therein only exists as water vapour.
A process for recovering elemental sulphur from an acid gas stream is proposed in U.S. Pat. No. 2,994,588 to Eickmeyer (issued Aug. 1, 1961). In that proposed process, a gas stream containing hydrogen sulphide and sulphur dioxide is reacted in a catalyst bed which is cooled with liquid sulphur. The liquid sulphur coolant includes a substantial quantity of absorbed hydrogen sulphide in order to lower its viscosity. In the Eickmeyer process absorption of H.sub.2 S by the liquid sulphur used to cool the bed plays an important part in the process. Thus, the bed is not cooled with substantially pure liquid sulphur, but with sulphur containing hydrogen sulphide. Furthermore some sulphur is removed from the top of the reactor as a vapour and the sulphur removed as a liquid is recycled.
While Eickmeyer did not disclose the pressures at which he proposed to operate his Claus catalytic reaction, the pressures are limited by several factors. Firstly, the hydrogen sulphide containing acid gas stream enters Eickmeyer's catalytic reactor from a conventional amine system. The pressures of the latter are limited to about 15 psig. Furthermore, an air blower is used to introduce air into the Claus combustion furnace from which the sulphur dioxide containing gases are introduced into the catalytic reactor; such an air blower is limited to pressures of about 30 psig (or 3 atmospheres absolute pressure). Eickmeyer does not suggest therefore operating the catalytic reactor at pressures above about 2 or at most 3 atmospheres.
Also, in Eickmeyer, the use of liquid sulphur to absorb H.sub.2 S means that the amount of liquid sulphur which is recirculated is very large compared to the amount of liquid sulphur produced by the process. In the first embodiment of Eickmeyer's process, the liquid sulphur is recirculated to an absorber vessel separate from the reactor. In the second embodiment, the absorption takes place in a packed bed at the bottom of the main reaction vessel. It is calculated, using the figures given by Eickmeyer for solubility of H.sub.2 S in liquid sulphur, that the first embodiment of his process would involve recirculating an amount of liquid sulphur more than 150 times greater than that produced by the process. It is not possible to calculate the amount recirculated in the second embodiment, but seemingly this would still be quite high. By contrast, the present invention uses recirculated sulphur only for cooling and not to absorb H.sub.2 S, and this invention uses much smaller amounts of recirculated sulphur, usually from 2 to 20 times the amount of net sulphur production (i.e., the total production of all the beds of the reactor).
It may also be noted that Eickmeyer does not suggest removing the sulphur as individual liquid streams from separate catalyst beds; nor does he suggest the possibility of using a high pressure continuous catalytic process for producing sulphur from the tail gas of a conventional Claus plant.
To our knowledge no one has hitherto demonstrated a catalytic process to operate efficiently and continuously at pressures considerably greater than atmospheric, while elemental sulphur is condensed in the catalyst bed from which it is removed as a liquid.